1. Field of the Invention
The present invention relates to the field of pumps for liquefied gases and more particularly to pumps for pumping pressurized carbon dioxide which is liquid at room temperature.
2. Description of the Prior Art
Pumps are known in the prior art for pumping liquefied carbon dioxide to increase the pressure of the carbon dioxide in order to create super-critical fluids using the carbon dioxide. The carbon dioxide to feed the pump is stored at a pressure at which it remains liquefied at room temperature. However, when the carbon dioxide is pumped, it may vaporize due to heat that it absorbs from the pump head.
In these prior art pumps, heat is generated during the pumping process by the compression of the carbon dioxide. This heat is transferred to the piston and pump cylinder causing the piston and pump cylinder to heat up. As the piston and pump cylinder heat up, at least a portion of the entering charge of carbon dioxide to be pumped is vaporized. Because the volume of a given mass of a gas is larger than the volume of the same mass of a liquid, the conversion of even a portion of the incoming charge of liquefied carbon dioxide to gaseous carbon dioxide causes a reduction in the available volume of liquefied carbon dioxide which can be pumped by the carbon dioxide pump.
U.S. Pat. Nos. 2,439,957; 2,439,958; and 2,440,216 show one approach which has been used to counteract the transfer of the heat generated from the compressed carbon dioxide to the piston and pump cylinder. These prior art pumps provide a sleeve enclosing the pump cylinder. In a second embodiment, a portion of the liquefied carbon dioxide that feeds the pump is blown through the chamber created between the sleeve and pump cylinder. The liquefied carbon dioxide cools the pump cylinder, thereby removing the heat generated during compression of the liquefied carbon dioxide. Because the carbon dioxide blown through the chamber is taken from the supply of liquefied carbon dioxide to be pumped, a portion of the supply of liquefied carbon dioxide is wasted. Other prior art pumps circulate alternative cooling fluids such as water, glycol and the like through the chamber between the sleeve and the pump cylinder to flush the chamber and thus cool the pump. The use of such cooling fluids increase the cost to use these pumps. Consequently, there is a need for an improved pump design which prevents the transfer of heat from the compressed liquefied carbon dioxide to the pump cylinder without exhausting any of the supply of the liquefied carbon dioxide or resorting to the additional need of other cooling fluids.